In the past, photovoltaic (PV) cells have been widely used to convert sunlight into electricity. A plurality of cells may be located behind a glass sheet to form a PV module. PV modules may receive a fraction of all the light that enters the glass, both direct sunlight and diffuse skylight. However, the efficiency of conversion of the total amount of incident solar energy is not high; for example, little more than 20% conversion may be achieved in current commercial PV modules. This limitation arises in part because sunlight comprises a broad range of wavelengths, and conventional PV modules use a single semiconductor type. While any given semiconductor may convert with high efficiency at a given characteristic wavelength, it is less efficient at other wavelengths. In the relatively inefficient spectral regions of any given PV cell, only a small amount of the available solar energy may be converted into electricity.
A PV module with higher overall efficiency may be preferred over a conventional module, provided the overall cost is not increased so much as to offset the efficiency gain. Sunlight may potentially be converted into electricity with higher overall efficiency than is possible with any one semiconductor, by dividing the solar spectrum and using the different parts to power PV cells using different semiconductors, each cell being illuminated preferentially by those parts of the spectrum which it converts with highest efficiency. One approach taken in the past used different semiconductors stacked on top of each other, forming a multijunction cell. In such a multijunction cell, different spectral bands separate out by absorption and conversion as sunlight travels down through the stack. However, this multijunction approach has typically been limited to expensive semiconductors and manufacturing techniques. To reduce the overall cost of energy generation by this approach, typically a small multijunction cell has been used in conjunction with optics to collect a large area of direct sunlight and strongly focus it onto the small cell area. However, in such configurations, the diffuse component of sunlight, which is typically between 20% and 40% of the total input, is nearly all lost, and in many cases system cost is increased because of the additional focusing optics and dual axis tracker required.
Other methods to use combinations of semiconductors of smaller area and/or of lower cost have been proposed, in which sunlight is first passed through optics which spatially separate the spectrum, directing different parts of the spectrum to different separated cells to better match their different spectral responses.
In prior art, Newton (“Opticks” 1704) provides a glass prism to separate sunlight into distinct spectral bands by refraction. Such refractive dispersion has the advantage of unambiguous wavelength separation, with angular deviation decreasing monotonically as wavelength is increased, but has the disadvantage that the angular separation is small. In a patent application (US 2010/0095999 A1) “Ultra-high efficiency multi junction solar cells using polychromatic diffractive concentrators”, inventor Menon proposes dispersion by a phase-plate and lens combination, the lenses focusing different wavelengths onto different laterally arranged cells. Diffraction by the phase plate gives higher angular spectral dispersion than a prism; however the design does not account for the fact that diffraction of any specific wavelength from the broad solar spectrum is generally in multiple orders, each being deflected (or directly transmitted) in a different direction. In another patent application, (US 20120318324 A1) “Laterally Arranged Multiple-Bandgap Solar Cells” 2012, inventors Ning and Caselli show laterally-arranged multiple bandgap solar cells and a notional depiction of dispersive concentrators positioned above to provide light to a surface of each of the cells, but do not provide specifics about the nature of the spectral separation, whether refractive or dispersive.
Zhang et al., Journal of Photonics for Energy, 2013, show a configuration with sunlight passing through a flat window of holographic lenses to PV cells of two different types. The lenses partially focus a band of the solar spectrum onto strips of cells of one type oriented perpendicular to the entrance window, while remaining light passes by to sheet of solar cells of another type oriented parallel to the entrance window.
In general, the prior art suffers from one or more of the following limitations: (1) it may not be configurable to have a large fraction of the solar energy entering a module directed to PV cells, thus losing area efficiency and driving up total area and cost; (2) the total cell area to convert all the entering sunlight may be significantly larger than the aperture area, thus driving up cell area and cost; (3) it may use a complex and thus expensive combination of dispersive elements and concentrating optics to obtain spectral separation; (4) the spectrally separating optics may direct some part of the solar spectrum to the targeted cell, while at the same time misdirect other parts of the spectrum—this is a common deficiency for diffractive spectral separation, when only a part of the solar spectrum is efficiently diffracted in any given one direction; and (5) they may use concentration and as a result lose some or most of the diffuse light component.